Pesticide compound are often applied in the form of a dilute aqueous composition in order to achieve a good interaction with the target organisms, such as plants, fungi and insects. However, most active ingredients that are used as pesticides, in particular pyrethroid compounds, are only sparingly or even insoluble in water, i.e. they usually have a water-solubility of not more than 5 g/l, often not more than 1 g/l and particularly not more than 0.1 g/l at 25° C./1013 mbar. Therefore, formulators are often confronted with difficulties in formulating pesticide compounds in stable formulations that can be easily diluted with water.
Pesticides having a limited solubility in water are often formulated as aqueous suspension concentrate (SC) which can be diluted with water for use in the field. Suspension concentrates are formulations, wherein the active ingredient is present in the form of finely divided solid particles, which are suspended in an aqueous dispersing medium utilizing surface-active compounds (surfactants), such as wetting agents, dispersants and rheological or suspending aids for stabilising the active ingredient particles in the dispersing medium. However, problems are often encountered with SC's as a result of settling during prolonged storage or storage at elevated temperatures, the resistance of settled particles to re-suspension and the formation of crystalline material upon storage. As a consequence, the formulations are difficult to handle and the bioefficacy may be inconsistent. Moreover, SC's are limited to actives that have a relatively high melting point. SC's are often not suitable for the formulating pyrethroid compounds.
An alternative for formulating active ingredients that are sparingly water-soluble are so-called emulsifiable concentrates (EC). In an EC the active ingredient is dissolved in a water-immiscible solvent (solubility usually <0.1 g/l), such as a hydrocarbon solvent, in particular an aromatic hydrocarbon, together with surfactants. EC's are usually stable solutions that can be diluted with water to form a milky oil-in-water (o/w) emulsion, containing the active ingredient in the droplets of the disperse phase. EC formulations have a considerable drawback in that they usually contain considerable amounts of hydrocarbon organic solvents which are not entirely satisfactory with regard to their ecological and toxicological properties. Since the droplets of the o/w emulsion that forms upon dilution with water, the bioefficacy of the active ingredient is sometimes not satisfactory. Moreover, separation of active ingredient may occur upon dilution with water, which may lead to inaccurate dosage and uneven bioefficacy.
Some of the deficiencies of EC-formulations can be overcome by microemulsion (ME) formulation technique. Microemulsions like conventional emulsions, also referred to as macro emulsions, are multiphase systems comprising a disperse phase and a continuous phase. In contrast to macro emulsions, the average particle (droplets) size (Z-average diameter as determined by light scattering) of the disperse phase in microemulsions is at least 5 times smaller than in macro emulsions and generally does not exceed 200 nm, while the average diameter of the droplets in macro emulsions is in μm range. Due to the small particle size (droplet size) of the disperse phase, microemulsions have a translucent appearance.
ME formulation of pesticide compounds are usually water based and additionally contain at least one surfactant and at least one cosolvent or cosurfactant, which is usually an organic solvent or a low molecular weight polyalkylene ether. By using ME formulations risks such as inflammability and toxicity, environmental concerns and costs can are reduced in comparison with EC techniques, because water is the main constituent. Due to the small particle size of the disperse phase containing the active ingredient, an increase in bioavailability can often be achieved. However, it is difficult to maintain the stability of ME formulation of active ingredients having a low water-solubility with respect to the droplet size and uniformity and crystallization of active ingredient may occur. Moreover, it is also difficult to maintain the droplet size stability when the ME formulation is diluted with water. This originates from low solubility of the active ingredient in water. However, a stable droplet size after dilution, i.e. maintaining a small droplet size, is important to achieve preferable biological activities. Therefore, much efforts were made in order to develop the stable water-based microemulsion formulation.
WO 88/07326 and WO 90/03112 describe microemulsions containing an insecticidal compound. The formulations contain a considerable amount of hydrocarbon solvent, surfactant and, as a cosurfactant, low molecular (ethylene oxide/propylene oxide) block copolymers of low HLB.
JP 3299812 describes herbicidal ME formulation which contain aromatic solvents and optionally ketones such cyclohexanone and ester of fatty acids such as methyl oleate as cosolvent.
WO 98/00010 discloses a microemulsion of insecticide compounds containing a hydrocarbon solvent and a cosolvent selected from C3-C12 alkanols and etheralkanols such as diethylene glycol monohexyl ether, diethylene glycol monobutyl ether and propylene glycol monobutyl ether.
U.S. Pat. No. 5,317,042 describes an aqueous ME of a pyrethroid insecticide, which contains a C1-C4-alkylpyrrolidone as a cosolvent.
By using lipophilic hydrocarbon solvents such as toluene, xylene or hydrocarbon mixtures such as Solvesso, Exxsol, Shellsol, etc., stable formulation could be obtained, which show reduced tendency of the active ingredient to crystallize. However, these solvents lead to increased skin sensitization. Replacing these solvents by less toxic polar solvents such as N-alkypyrrolidones, C1-C3-alkanols, ketones, ethylene glycol, propylene glycol, di-C2-C3-alkylene glycols etc. leads to reduced stability of the dilution originating from reduced solubility of active ingredient in the solvent under the diluted conditions. The aforementioned problems are particularly pronounced in case of pyrethroid compounds.
Therefore, there is an ongoing need for ME formulations of pyrethroid compounds which are uniform and stable. In particular, the formulation should be stable upon dilution with water, i.e. they should provide stable size distribution of small droplets after dilution with water. Moreover, they should provide reduced tendency of the active ingredient to crystallize, in particular after dilution of the formulation with water. Moreover the ME formulation should maintain its liquid state at low temperatures, i.e. at temperature below 0° C. It is highly desirable to achieve these objects without the use of hydrocarbon solvent, in order to increase ecological compatibility and to reduce the risk of skin irritation. In particular ME formulations are required which provide stable formulations of pyrethroid esters, in particular pyrethroid esters having a biphenylether moiety and especially flucythrinate or alpha cypermethrin or mixtures thereof.